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First published online June 16, 2005
Journal of Experimental Biology 208, 2549-2554 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01665

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Emperor penguins adjust swim speed according to the above-water height of ice holes through which they exit

Katsufumi Sato^1,*, Paul J. Ponganis², Yoshiaki Habara³ and Yasuhiko Naito⁴

¹ International Coastal Research Center, Ocean Research Institute, University of Tokyo, 2-106-1 Akahama, Otsuchi, Iwate 028-1102, Japan
² Center for Marine Biotechnology and Biomedicine, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0204, USA
³ Laboratory of Physiology, Department of Biomedical Sciences, Graduate School of Veterinary Medicine, Hokkaido University, North 18, West 9, Sapporo 060-0818, Japan
⁴ Biologging Institute, 2-31-10, Rex Yushima 301, Yushima Bunkyo, Tokyo 113-0034, Japan

View larger version (24K): [in a new window]   Fig. 1. Schematic diagram of exit options for the penguins. As shown on the left, the bird could climb onto the ice if the center of gravity reached the height of the ice around the hole, or the exit speed was sufficient to project the bird onto the ice (the right-hand bird).

View larger version (18K): [in a new window]   Fig. 2. Typical profiles for last 20 s before exit through the ice hole in 6th deployment by bird no. 314. The closed circle indicates the maximum swim speed before exit. Horizontal bars indicate strokes based on acceleration.

View larger version (22K): [in a new window]   Fig. 3. Relationship between above-water height and the maximum speed before exit. Observed data (386 exits in 13 deployments by nine birds are pooled) are represented by closed circles (successful exit) and crosses (failed exit). The solid curve is the theoretical line for initial vertical speed to reach the height, assuming a take-off angle () of 90° (see text for details). If penguins adopt non-vertical body angles, higher swim speeds were needed to obtain the necessary vertical speeds for the height, which are represented by dotted curves for various take-off angles.

View larger version (28K): [in a new window]   Fig. 4. Box plots for the comparison in the maximum speed before exit between lower (white bars) and higher (gray bars) holes for each deployment. The differences in the above-water height of holes were less than 10 cm in four deployments (from 7th to 10th). *P<0.05; **P<0.01; ***P<0.001; two-tailed Mann-Whitney U tests; n.s., not significant.

View larger version (17K): [in a new window]   Fig. 5. An example of the last 12 s before exit through the ice hole in the 8th deployment by bird no. 429. The closed circle indicates the maximum swim speed before exit. Horizontal bars indicate strokes based on acceleration.